This invention relates to an Automatic Track Finding (ATF) controller for Digital Audio Tape Recorder (hereinafter referred to as "DAT"), particularly capable of performing the ATF operation by an offset adjustment of a level difference between two separate channels after digital conversion of a pilot signal detected.
Generally, in DAT, digital audio data and ATF track pattern are recorded onto a tape as in FIG. 1. The DAT system reproduces each audio data which is recorded on an odd frame address track and an even frame address track, with two heads, A and B. In the DAT tape shown in FIG. 1, (A) is plus azimuth track and (B) is minus azimuth track both of which are areas for writing PCM audio data, IBG is a guide area to separate PCM data from ATF signal. In such a configuration of ATF signal, f1 is a recording area of 130.67-KHz pilot signal, f2 is a recording area of 522.67-KHz first synchronization signal SYNC1, f3 is a recording area of 784.00-KHz second synchronization signal SYNC2 and f4 is a recording area of 1.568-KHz tape erase signal.
Referring to the above described tape configuration, there will be understood that the first synchronization signal f2 is recorded on the track (A) and the second synchronization signal f3 is recorded in track (B), wherein since width of head is 1.5 times as large as that of track on the tape, when the head passes a certain synchronization signal f2 or f3, it overlaps the pilot signal f1 of a first adjacent track, and after passing two blocks, it overlaps the pilot signal f1 of a second adjacent track.
In the DAT using the tapes formatted as above described, the running of tape and tracking of drum are controlled by using said signals. That is, since heads of a conventional DAT are constructed with two heads (A and B), there arises in reproduction a level difference upon pilot signals between two channels according to the head characteristics. To compensate this difference, there has been used a known circuit shown in FIG. 2, wherein among RF signals reproduced by the heads, a pilot signal on each channel is detected by low-pass filter 21 and the detected pilot signal is amplified by an amplifier 22. Here, feedback output controlled by switches SWA,SWB connected through feedback resistors R2,R1 is supplied to a negative input of the amplifier 22, wherein SWA is a selection switch for a channel A and SWB is that for a channel B. And R1, C1 and VR1 are for determining a time constant to adjust the gain of channel A, while R2, C2 and VR2 is for channel B. So, the level difference due to the characteristics of heads A and B is adjusted by the switches SWA, SWB. The output of Amplifier 22 which is gain-adjusted by the above manner is inputted to a sample and hold circuit through a peak hold section after being rectified at a rectifier 23. Referring to FIG. 3, a pilot signal for each track is detected through the heads on time period 3a, the crosstalk of pilot signal on the first adjacent track (upper track) is detected on time period 3b, and the crosstalk of pilot signal on the second adjacent track (lower track) is detected on time period 3c. Therefore, at the time period 3b, the first sample and hold circuit 25 holds the crosstalk of pilot signal of the upper track generated at the peak hold section 24, while the time period 3c, the peak hold 24 holds the crosstalk of pilot signal of the lower track. Thus, an adder 26 evaluates an output difference of the first sample and hold circuit 25 from output of the peak hold section 24, and passes it to the second sample and hold circuit 27. Here, the signal 3d held at the second sample and hold circuit 27 is so called an ATF error voltage by which the controller of DAT performs the function of AFT. But there have been some problems in adjusting the various parameters and the time constant for compensation of level difference in the assembly process, since in the conventional ATF control circuit, the adjustment of level difference due to the head characteristics of two channels A and B can be done by applying two adjusting points and the compensation of level difference may be made by generating its compensating time constant through an analog circuit for each channel, simultaneously.